seki has asked for the wisdom of the Perl Monks concerning the following question:

Hi Monks!

I use often the localtime() / strftime() couple, but while looking at my collection of one-liners tools, I am puzzled by an epoch translator that displays local time without specifying a format.

If I eval the following code,

perl -le'print localtime 1503403724'

the result is not surprisingly 

4481422711722331

but the following command line filter

echo 1503403724 | perl -pe's/([\d.]+)/localtime $1/e;'

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displays the formated 

Tue Aug 22 14:08:44 2017

Is there some untold magic with the inline loop parameter, or with the executable regex?

For the record, the onle-liner is originaly used to translate some log files with a time-stamp on each line in the form of an epoch date instead of a readable date format. Example:

cat var/nagios.log | perl -pe's/([\d.]+)/localtime $1/e;' | less

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The best programs are the ones written when the programmer is supposed to be working on something else. - Melinda Varian

Replies are listed 'Best First'.
Re: one liner automagic strftime()
by hdb (Monsignor) on Aug 22, 2017 at 13:07 UTC

    In the substitution operator the right hand side is evaluated in scalar context and thus localtime returns the formatted date as a string as per documentation: 

    use strict;
use warnings;

my @x = localtime 1503403724;
print "@x\n";

my $x = localtime 1503403724;
print "$x\n";

my $z = "ccc";
$z =~ s/ccc/wantarray()?"list":"scalar"/e;
print "$z\n";

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      Arf. So no magic, just the list / scalar context. I have always missed that paragraph of the localtime() perldoc... Thanks.

      The best programs are the ones written when the programmer is supposed to be working on something else. - Melinda Varian
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        Difficult to say as "Any sufficiently advanced technology is indistinguishable from magic." (Arthur C. Clarke)

        Perl surely falls into that category...

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Re: one liner automagic strftime()
by Laurent_R (Canon) on Aug 22, 2017 at 22:04 UTC
    Try this: 
    $ perl -le'print scalar localtime  1503403724'
Tue Aug 22 14:08:44 2017

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